1. Field of the Invention
The present invention relates to control of a dimmable discharge lamp, and more specifically to generation of a dimming current for a fluorescent lamp.
2. Description of the Related Art
The electronic ballast for fluorescent lamp dimming control could use either a series LC resonant series loaded circuit, a series resonance parallel loaded circuit, or a series parallel resonance circuit, controlled by either the frequency or the duty cycle of input voltage pulses. The existing duty cycle control employs symmetrically chopped pulses. The series LC resonant series loaded and series parallel resonance circuits are not commonly used for electronic ballast because they behave like a band-pass filter, and so cannot satisfy the high gain required at high impedance for ignition and low load dimming.
The most common type of conventional electronic ballast uses a series resonance parallel loaded circuit, the structure of which is shown in FIG. 1A. They behave like a low-pass filter and show a high gain at high impedance that is required during ignition and low dimming by the fluorescent lamp. The input of the ballast comes from a DC source that could be a pre-stage power factor correction (PFC) universal boost unit. Switching elements S1 and S2 turn on and off in response to a signal from a controller 10 to convert the DC voltage into an AC voltage. The controller 10 controls states of the switching elements S1 and S2, and thus the waveform of the AC voltage, in accordance with a desired dimming level from a dimmer 11. That is, by adjusting the dimmer 11, the current Ilamp flowing through a fluorescent lamp 12 can be changed, and the light output of the fluorescent lamp 12 can be varied. A resonance circuit, comprising an inductor L and a capacitor C1, is formed between the switching stage, including switching elements S1 and S2, and the fluorescent lamp 12. A capacitor C2 blocks DC voltage to the fluorescent lamp 12.
The main relations among the signals in the circuit are as follows:
                              i          L                =                              i            lamp                    +                      C            ⁢                                                  ⁢            1            ⁢                          (                                                dv                  C1                                ⁢                                  /                                ⁢                dt                            )                                                          (        1        )                                          v          c1                =                                            V              in                        -                          L              ⁡                              (                                                      di                    L                                    ⁢                                      /                                    ⁢                  dt                                )                                              =                                                    1                                  C                  ⁢                                                                          ⁢                  1                                            ⁢                              ∫                                                      (                                                                  i                        L                                            -                                              i                        lamp                                                              )                                    ⁢                                      ⅆ                    t                                                                        +                                          V                c1                            ⁡                              (                IC                )                                                                        (        2        )                                          v          c2                =                                            V              in                        -                          L              ⁡                              (                                                      di                    L                                    ⁢                                      /                                    ⁢                  dt                                )                                      -                                          R                lamp                            ·                              i                lamp                                              =                                                    1                                  C                  ⁢                                                                          ⁢                  2                                            ⁢                              ∫                                                      i                    lamp                                    ⁢                                      ⅆ                    t                                                                        +                                          V                c2                            ⁡                              (                IC                )                                                                        (        3        )            
wherein Vc1(IC) is the Initial Condition of voltage across C1, and Vc2(IC) is the Initial Condition of voltage across C2.
As shown in FIG. 1B, the series resonance parallel loaded circuit behaves as a low-pass filter. The fundamental frequency of the square input pulse would be in the pass band of the network and higher harmonics mainly would be attenuated. The transfer function of the series resonance parallel loaded circuit is:
            G      p        ⁡          (      jw      )        =                    Vo        ⁡                  (          jω          )                            Vi        ⁡                  (          jω          )                      =          1                                                  (                              1                -                                                      (                                          ω                      ωo                                        )                                    2                                            )                        2                    +                                    (                              ω                ωoQp                            )                        2                              
wherein,ωo=1/√LC1Qo=R/Lωo=RCωo 
The series resonant parallel loaded ballast with double switch choppers at the DC output of the PFC boost is preferred over other conventional ballasts, because it is adjustable with high voltage requirement at high impedance of ignition, is short circuit proof, and its voltage increases in high impedance and low load during dimming.
According to one of the conventional approaches, the controller 10 changes the current Ilamp by controlling the frequency fsw at which the switching elements S1 and S2 turn on and off. The frequency control is used with a fixed duty cycle D=50%. Square pulses of Vin to the ballast are assumed to be DC modulated with a sine wave of switching frequency. The DC component shifts the AC voltage across C1 and is blocked by C2. The average DC voltage, Vav=Vdc/2, remains constant in all loads and a uniform resonance sine wave is assumed over the whole period.
As shown in FIG. 2, at higher loads the current Ilamp increases with the decrease of the frequency fsw. However, in some threshold of low dim range, the curve becomes flat, and the light output of the fluorescent lamp 12 cannot be effectively adjusted by changing the frequency fsw. This threshold depends on the lamp characteristic, input/output voltage, as well as the optimized component selection of C1 and L.
Another disadvantage of conventional frequency control dimming is that in this flat area of low load control the ballast is too sensitive to the frequency changes. When the frequency fsw is raised quickly, the response of the circuit is so fast that the ballast becomes unstable. Thus, conventionally, only gradual dimming could be used.
According to another conventional approach, the controller 10 changes the current Ilamp by controlling the duty cycle Dsw of the switching elements S1 and S2. As shown in FIG. 3, dimming is achieved by reducing pulse width of both switches symmetrically, and symmetric charge/discharge time is used to avoid DC voltage drop. However, there is a gap between the turn on (or close) time of the two switching elements, which may cause a discontinuous conduction mode in a resonant tank circuit at low dimming, and high peak current that lowers the efficiency.
Therefore, it would be advantageous to provide a method and apparatus for effective and efficient control of the dimming of the fluorescent lamp.